Vaccinia-related Kinase Research Articles

Association of the VRK2 gene rs3732136 polymorphism with schizophrenia in a Northwest Chinese Han population.

Previous studies have found that the vaccinia related kinase 2 gene (VRK2) polymorphism was associated with schizophrenia (SCZ) in the worldwide population. This association was further supported by VRK2 mRNA expression patterns and brain structure variations. Here, we analyzed four single nucleotide polymorphisms (SNPs) of the VRK2 gene in a total population of 893 samples, consisting of 360 patients with SCZ and 533 healthy controls of Han Chinese descent using the SNPscan method. Single SNP, haplotype, and gender-specific association analyses were performed. We found that rs3732136 was significantly associated with SCZ (P = 0.042; odds ratio = 1.25; 95% confidence interval = 1.01-1.55). Further genotype and haplotype association analyses suggested a similar pattern. Our data provide preliminary evidence that the VRK2 gene might play a major role in the development of SCZ in the Northwest Chinese Han population.

The changes of vaccinia related kinase 1 in grafted heart after rat heart transplantation.

To assess the expression and significance of vaccinia-related kinase 1 (VRK1) after rat heart transplantation. Lewis and Wistar rats weighing 250 to 300 g were used as donors and recipients. Allografts were from Wistar transplanted into Lewis, and isografts were transplanted from Lewis into Lewis. Grafts were harvested at 1, 3, 5, and 7 days after transplantation. We performed Western Blot of heart tissues after cardiac transplantation. To analyze VRK1 express between the isografts and allografts for immunohistochemical staining. At 5th day after heart transplantation use related cytokines VRK1 for immunohistochemical. We used double immunofluorescent staining on transverse cryosections of graft tissues by co-labeling with different markers, including those for VRK1, activate caspase-3, α-actinin, VCAM-1, CD4. Compared with rare expression in syngeneic Lewis rat hearts, VRK1 protein level in allogeneic hearts were detected at various survival times after heterotopic heart transplantation, which observably expressed on day 5 postoperative. In addition, we examined the expression of activate caspase-3 in allogeneic hearts, which has a similar expression with VRK1. Immunohistochemical and immunofluorescent method displayed that VRK1 was widely expressed in cytoplasm of cardiac tissue and activate caspase-3 was also expressed in cardiomyocytes. However, the VRK1 wasn't express in inflammation. The VRK1 expression has increased after heart transplantation in allograft and isograft, and VRK1 may play a significant role in myocardial apoptosis after heterotopic heart transplantation in rats.

Brazilin Isolated from Caesalpinia sappan suppresses nuclear envelope reassembly by inhibiting barrier-to-autointegration factor phosphorylation.

To date, many anticancer drugs have been developed by directly or indirectly targeting microtubules, which are involved in cell division. Although this approach has yielded many anticancer drugs, these drugs produce undesirable side effects. An alternative strategy is needed, and targeting mitotic exit may be one alternative approach. Localization of phosphorylated barrier-to-autointegration factor (BAF) to the chromosomal core region is essential for nuclear envelope compartment relocalization. In this study, we isolated brazilin from Caesalpinia sappan Leguminosae and demonstrated that it inhibited BAF phosphorylation in vitro and in vivo. Moreover, we demonstrated direct binding between brazilin and BAF. The inhibition of BAF phosphorylation induced abnormal nuclear envelope reassembly and cell death, indicating that perturbation of nuclear envelope reassembly could be a novel approach to anticancer therapy. We propose that brazilin isolated from C. sappan may be a new anticancer drug candidate that induces cell death by inhibiting vaccinia-related kinase 1-mediated BAF phosphorylation.

Luteolin Suppresses Cancer Cell Proliferation by Targeting Vaccinia-Related Kinase 1

Uncontrolled proliferation, a major feature of cancer cells, is often triggered by the malfunction of cell cycle regulators such as protein kinases. Recently, cell cycle-related protein kinases have become attractive targets for anti-cancer therapy, because they play fundamental roles in cellular proliferation. However, the protein kinase-targeted drugs that have been developed so far do not show impressive clinical results and also display severe side effects; therefore, there is undoubtedly a need to investigate new drugs targeting other protein kinases that are critical in cell cycle progression. Vaccinia-related kinase 1 (VRK1) is a mitotic kinase that functions in cell cycle regulation by phosphorylating cell cycle-related substrates such as barrier-to-autointegration factor (BAF), histone H3, and the cAMP response element (CRE)-binding protein (CREB). In our study, we identified luteolin as the inhibitor of VRK1 by screening a small-molecule natural compound library. Here, we evaluated the efficacy of luteolin as a VRK1-targeted inhibitor for developing an effective anti-cancer strategy. We confirmed that luteolin significantly reduces VRK1-mediated phosphorylation of the cell cycle-related substrates BAF and histone H3, and directly interacts with the catalytic domain of VRK1. In addition, luteolin regulates cell cycle progression by modulating VRK1 activity, leading to the suppression of cancer cell proliferation and the induction of apoptosis. Therefore, our study suggests that luteolin-induced VRK1 inhibition may contribute to establish a novel cell cycle-targeted strategy for anti-cancer therapy.

Effects of VRK2 (rs2312147) on white matter connectivity in patients with schizophrenia.

Recent genome-wide association studies of schizophrenia reported a novel risk variant, rs2312147 at vaccinia-related kinase 2 gene (VRK2), in multiple Asian and European samples. However, its effect on the brain structure in schizophrenia is little known. We analyzed the brain structure of 36 schizophrenia patients and 18 healthy subjects with regard to rs2312147 genotype groups. Brain magnetic resonance scans for gray matter (GM) and white matter (WM) analysis, and genotype analysis for VRK2 rs2312147, were conducted. The Positive and Negative Syndrome Scale and the Digit Symbol Test were assessed for schizophrenia patients. There was no significant difference in either GM volume or WM connectivity with regard to rs2312147 genotype in healthy subjects. In contrast, we found significant differences in the WM connectivity between rs2312147 CC and CT/TT genotype groups of schizophrenia patients. The related brain areas included the splenium of corpus callosum, the left occipital lobe WM, the internal capsule (left anterior limb and right retrolenticular part), the bilateral temporal lobe WM, the left fornix/stria terminalis, the left cingulate gyrus WM, and the left parietal lobe WM. Voxelwise correlation analysis revealed that the Digit Symbol Test scores (age corrected) correlated with the fractional anisotropy in WM tracts that previously showed significant group differences between the CT/TT and CC genotypes in the rs2312147 CT/TT genotype group, while no significant correlation was found in the CC genotype group. Our data may provide evidence for the effect of VRK2 on WM connectivity in patients with schizophrenia.

Vaccinia-Related Kinase 2 Mediates Accumulation of Polyglutamine Aggregates via Negative Regulation of the Chaperonin TRiC

Misfolding of proteins containing abnormal expansions of polyglutamine (polyQ) repeats is associated with cytotoxicity in several neurodegenerative disorders, including Huntington's disease. Recently, the eukaryotic chaperonin TRiC hetero-oligomeric complex has been shown to play an important role in protecting cells against the accumulation of misfolded polyQ protein aggregates. It is essential to elucidate how TRiC function is regulated to better understand the pathological mechanism of polyQ aggregation. Here, we propose that vaccinia-related kinase 2 (VRK2) is a critical enzyme that negatively regulates TRiC. In mammalian cells, overexpression of wild-type VRK2 decreased endogenous TRiC protein levels by promoting TRiC ubiquitination, but a VRK2 kinase-dead mutant did not. Interestingly, VRK2-mediated downregulation of TRiC increased aggregate formation of a polyQ-expanded huntingtin fragment. This effect was ameliorated by rescue of TRiC protein levels. Notably, small interference RNA-mediated knockdown of VRK2 enhanced TRiC protein stability and decreased polyQ aggregation. The VRK2-mediated reduction of TRiC protein levels was subsequent to the recruitment of COP1 E3 ligase. Among the members of the COP1 E3 ligase complex, VRK2 interacted with RBX1 and increased E3 ligase activity on TRiC in vitro. Taken together, these results demonstrate that VRK2 is crucial to regulate the ubiquitination-proteosomal degradation of TRiC, which controls folding of polyglutamine proteins involved in Huntington's disease.

Vaccinia-related kinase 1 (VRK1) confers resistance to DNA-damaging agents in human breast cancer by affecting DNA damage response

Vaccinia-related kinase 1 (VRK1) belongs to a group of sixteen kinases associated to a poorer prognosis in human breast carcinomas, particularly in estrogen receptor positive cases based on gene expression arrays. In this work we have studied the potential molecular mechanism by which the VRK1 protein can contribute to a poorer prognosis in this disease. For this aim it was first analyzed by immunohistochemistry the VRK1 protein level in normal breast and in one hundred and thirty six cases of human breast cancer. The effect of VRK1 to protect against DNA damage was determined by studying the effect of its knockdown on the formation of DNA repair foci assembled on 53BP1 in response to treatment with ionizing radiation or doxorubicin in two breast cancer cell lines. VRK1 protein was detected in normal breast and in breast carcinomas at high levels in ER and PR positive tumors. VRK1 protein level was significantly lower in ERBB2 positive cases. Next, to identify a mechanism that can link VRK1 to poorer prognosis, VRK1 was knocked-down in two breast cancer cell lines that were treated with ionizing radiation or doxorubicin, both inducing DNA damage. Loss of VRK1 resulted in reduced formation of DNA-damage repair foci complexes assembled on the 53BP1 scaffold protein, and this effect was independent of damaging agent or cell type. This observation is consistent with detection of high VRK1 protein levels in ER and PR positive breast cancers. We conclude that VRK1 can contribute to make these tumors more resistant to DNA damage-based therapies, such as ionizing radiation or doxorubicin, which is consistent with its association to a poor prognosis in ER positive breast cancer. VRK1 is potential target kinase for development of new specific inhibitors which can facilitate sensitization to other treatments in combination therapies; or alternatively be used as a new cancer drugs.

Dephosphorylation of Barrier-to-autointegration Factor by Protein Phosphatase 4 and Its Role in Cell Mitosis

Barrier-to-autointegration factor (BAF or BANF1) is highly conserved in multicellular eukaryotes and was first identified for its role in retroviral DNA integration. Homozygous BAF mutants are lethal and depletion of BAF results in defects in chromatin segregation during mitosis and subsequent nuclear envelope assembly. BAF exists both in phosphorylated and unphosphorylated forms with phosphorylation sites Thr-2, Thr-3, and Ser-4, near the N terminus. Vaccinia-related kinase 1 is the major kinase responsible for phosphorylation of BAF. We have identified the major phosphatase responsible for dephosphorylation of Ser-4 to be protein phosphatase 4 catalytic subunit. By examining the cellular distribution of phosphorylated BAF (pBAF) and total BAF (tBAF) through the cell cycle, we found that pBAF is associated with the core region of telophase chromosomes. Depletion of BAF or perturbing its phosphorylation state results not only in nuclear envelope defects, including mislocalization of LEM domain proteins and extensive invaginations into the nuclear interior, but also impaired cell cycle progression. This phenotype is strikingly similar to that seen in cells from patients with progeroid syndrome resulting from a point mutation in BAF.

Solution structure of human full-length vaccinia related kinase 1 (VRK1)

Solution structure of human full-length vaccinia related kinase 1 (VRK1)

Mutations inVRK1Associated With Complex Motor and Sensory Axonal Neuropathy Plus Microcephaly

Patients with rare diseases and complex clinical presentations represent a challenge for clinical diagnostics. Genomic approaches are allowing the identification of novel variants in genes for very rare disorders, enabling a molecular diagnosis. Genomics is also revealing a phenotypic expansion whereby the full spectrum of clinical expression conveyed by mutant alleles at a locus can be better appreciated. To elucidate the molecular cause of a complex neuropathy phenotype in 3 patients by applying genomic sequencing strategies. Three affected individuals from 2 unrelated families presented with a complex neuropathy phenotype characterized by axonal sensorimotor neuropathy and microcephaly. They were recruited into the Centers for Mendelian Genomics research program to identify the molecular cause of their phenotype. Whole-genome, targeted whole-exome sequencing, and high-resolution single-nucleotide polymorphism arrays were performed in genetics clinics of tertiary care pediatric hospitals and biomedical research institutions. Whole-genome and whole-exome sequencing identified the variants responsible for the patients' clinical phenotype. We identified compound heterozygous alleles in 2 affected siblings from 1 family and a homozygous nonsense variant in the third unrelated patient in the vaccinia-related kinase 1 gene (VRK1). In the latter subject, we found a common haplotype on which the nonsense mutation occurred and that segregates in the Ashkenazi Jewish population. We report the identification of disease-causing alleles in 3 children from 2 unrelated families with a previously uncharacterized complex axonal motor and sensory neuropathy accompanied by severe nonprogressive microcephaly and cerebral dysgenesis. Our data raise the question of whether VRK1 mutations disturb cell cycle progression and may result in apoptosis of cells in the nervous system. The application of unbiased genomic approaches allows the identification of potentially pathogenic mutations in unsuspected genes in highly genetically heterogeneous and uncharacterized neurological diseases.

VRK2 identifies a subgroup of primary high-grade astrocytomas with a better prognosis

BackgroundMalignant astrocytomas are the most common primary brain tumors and one of the most lethal among human cancers despite optimal treatment. Therefore, the characterization of molecular alterations underlying the aggressive behavior of these tumors and the identification of new markers are thus an important step towards a better patient stratification and management.Methods and resultsVRK1 and VRK2 (Vaccinia-related kinase-1, -2) expression, as well as proliferation markers, were determined in a tissue microarray containing 105 primary astrocytoma biopsies. Kaplan Meier and Cox models were used to find clinical and/or molecular parameters related to overall survival. The effects of VRK protein levels on proliferation were determined in astrocytoma cell lines. High levels of both protein kinases, VRK1 or VRK2, correlated with proliferation markers, p63 or ki67. There was no correlation with p53, reflecting the disruption of the VRK-p53-DRAM autoregulatory loop as a consequence of p53 mutations. High VRK2 protein levels identified a subgroup of astrocytomas that had a significant improvement in survival. The potential effect of VRK2 was studied by analyzing the growth characteristics of astrocytoma cell lines with different EGFR/VRK2 protein ratios.ConclusionHigh levels of VRK2 resulted in a lower growth rate suggesting these cells are more indolent. In high-grade astrocytomas, VRK2 expression constitutes a good prognostic marker for patient survival.

Rewiring of human lung cell lineage and mitotic networks in lung adenocarcinomas

Analysis of gene expression patterns in normal tissues and their perturbations in tumors can help to identify the functional roles of oncogenes or tumor suppressors and identify potential new therapeutic targets. Here, gene expression correlation networks were derived from 92 normal human lung samples and patient-matched adenocarcinomas. The networks from normal lung show that NKX2-1 is linked to the alveolar type 2 lineage, and identify PEBP4 as a novel marker expressed in alveolar type 2 cells. Differential correlation analysis shows that the NKX2-1 network in tumors includes pathways associated with glutamate metabolism, and identifies Vaccinia-related kinase (VRK1) as a potential drug target in a tumor-specific mitotic network. We show that VRK1 inhibition cooperates with inhibition of PARP signaling to inhibit growth of lung tumor cells. Targeting of genes that are recruited into tumor mitotic networks may provide a wider therapeutic window than that seen by inhibition of known mitotic genes.

Structural insights into the VRK1‐mediated histone phosphorylation and its regulation by mH2A1, a histone H2A variant

Vaccinia related kinase 1 (VRK1), a mitotic protein kinase, is involved in cell proliferation, cell cycle, neuronal development, and gamatogenesis. VRK1‐induced phosphorylation of histone H3 results in nuclear condensation facilitating mitosis during cell division. To understand the VRK1's role in cell cycle transition, we studied the VRK1‐mediated histone phosphorylation and catalytic modulation by regulatory molecules. Interestingly, the VRK1 catalytic activity increases in late G2 and M phases of cell cycle and subsequently decreases at the end of anaphase. We showed that mH2A1, which is a histone H2A variant, suppresses the VRK1 activity during interphase and influences the cell cycle progression. Our NMR solution structure of human VRK1 revealed that its overall fold is similar to other VRK family kinases, VRK2 and VRK3. Despite their overall structural similarity, notable differences in their local structures were observed, in particular in the catalytic loop, activation loop and the non‐catalytic fragment at the C‐terminus of VRK1. The C‐terminal region orients around the catalytic site to be essential for its structural stability and biological activity. We showed that mH2A1 binds to the C‐terminal fragment and inhibits VRK1's catalytic activity. Hence our findings might provide structural insights into the chromatin epigenetic regulation through the VRK1‐mediated histone phosphorylation.

Role of Bivalent Cations in Structural Stabilities of New Drug Targets ——Vacciniarelated Kinases (VRK) from Molecular Dynamics Simulations

Protein kinases, which play an important role in the regulation of the majority of cellular processes, especially those involved in cellular signal transduction, by catalyzing the phosphorylation of specific proteins, are the attractive targets of drug design in pharmaceuticals industry. Interestingly, up to 10% of proteins in the human kinome termed pseudokinases are predicted to be enzymatically inactive, but are still pivotal in regulating diverse cellular processes and thus may be a potential therapeutic target to a certain extent. To study the underlying molecular mechanisms, molecular dynamics simulations were performed to investigate the role of bivalent cations Mg²⁺ and Mn²⁺ in the structural stabilities and dynamical behaviors of vaccinia related kinase 3 (VRK3), which was the first solved crystal structure of the pseudokinase, and that of its closest active relatives VRK1 and VRK2. Toward this end, a series of molecular dynamics simulations have been performed with different divalent cations binding modes in the active site. The simulations suggested that the binding of Mg²⁺ in the active site played a key structural role in the stabilization of VRK1 and VRK2, and Mn²⁺ was slightly required for VRK2. By contrast, the pseudokinase VRK3 was well ordered with lower RMSD values indicating it was rigid and very stable regardless of whether the bivalent cations were bound or not during the simulations. The present study provided evidence for the role of bivalent cations in structural stabilities of VRKs and the proposed simulation model reconciled the interpretation of available experimental structural and thermal denaturation assay data. These results gave us further information on the dynamical behaviors of the active site of VRKs and suggested a mechanism of regulation of their structural stabilities, and might provide a starting point for the more detailed follow- up investigation of drug design.

A Novel Approach to Predict Schizophrenia Disease: Conceptual Framework

A multilayer perceptrons (MLP) is proposed to aid in early detection of schizophrenia in this study. Schizophrenia is a disabling illness with unknown pathogenesis. Clinical studies have shown that there is a genetic contribution to the pathogenesis of schizophrenia. The transcription factor 4 (TCF4) gene, vaccinia-related kinase 2 (VRK2) gene as well as other five genes which were newly-identified loci of 1p21.3, 2p32.3, 8pp23.3, 8q21.3 and 10q24.32-p22.1 are among the most prominent susceptibility genes for schizophrenia. It has been hypothesized that TCF4 affects normal brain development and it has been related to different forms of neurodevelopmental disorders. Despite numerous linkage and association analyses (haplotype and meta analyses), the seven gene polymorphisms that contribute to schizophrenia remain to be elucidated. Using a haplotype block-based gene-tagging approach we will conduct an association study of thirty single nucleotide polymorphisms (SNPs) of the above seven gene polymorphisms. Artificial Neural Networks (ANNs) are widely used in prediction application. This research work will demonstrate that the MLP based prediction of schizophrenia which used genetic data (simple combination of thirty genotypic frequencies) of thirty SNPs to improve the detection possibility and accuracy with high consistency. This ANN is expected to exhibit good performance in the prediction of schizophrenia.

Mitogen-activated protein kinase phosphatase 2 regulates histone H3 phosphorylation via interaction with vaccinia-related kinase 1

Mitogen-activated protein kinase phosphatase 2 (MKP2) is a member of the dual-specificity MKPs that regulate MAP kinase signaling. However, MKP2 functions are still largely unknown. In this study, we showed that MKP2 could regulate histone H3 phosphorylation under oxidative stress conditions. We found that MKP2 inhibited histone H3 phosphorylation by suppressing vaccinia-related kinase 1 (VRK1) activity. Moreover, this regulation was dependent on the selective interaction with VRK1, regardless of its phosphatase activity. The interaction between MKP2 and VRK1 mainly occurred in the chromatin, where histones are abundant. We also observed that the protein level of MKP2 and its interaction with histone H3 increased from G1 to M phase during the cell cycle, which is similar to the VRK1 profile. Furthermore, MKP2 specifically regulated the VRK1-mediated histone H3 phosphorylation at M phase. Taken together, these data suggest a novel function of MKP2 as a negative regulator of VRK1-mediated histone H3 phosphorylation.

Backbone 1H, 13C and 15N resonance assignments of human vaccinia-related kinase 1 (VRK1)

Vaccinia-related kinase 1 (VRK1) is one of the mitotic kinases which play key roles in cell cycle control and chromatin modifications. To understand the biological role of the kinase and gain insights into its catalytic mechanism, we performed NMR assignments of catalytically active form of VRK1 with 361 amino acids residues. Here, we present the backbone NMR resonance assignments of the kinase.

Human VRK2 (Vaccinia-related Kinase 2) Modulates Tumor Cell Invasion by Hyperactivation of NFAT1 and Expression of Cyclooxygenase-2

Human VRK2 (vaccinia-related kinase 2), a kinase that emerged late in evolution, affects different signaling pathways, and some carcinomas express high levels of VRK2. Invasion by cancer cells has been associated with NFAT1 (nuclear factor of activated T cells) activation and expression of the COX-2 (cyclooxygenase 2) gene. We hypothesized that VRK proteins might play a regulatory role in NFAT1 activation in tumor cells. We demonstrate that VRK2 directly interacts and phosphorylates NFAT1 in Ser-32 within its N-terminal transactivation domain. VRK2 increases NFAT1-dependent transcription by phosphorylation, and this effect is only detected following cell phorbol 12-myristate 13-acetate and ionomycin stimulation and calcineurin activation. This NFAT1 hyperactivation by VRK2 increases COX-2 gene expression through the proximal NFAT1 binding site in the COX-2 gene promoter. Furthermore, VRK2A down-regulation by RNA interference reduces COX-2 expression at transcriptional and protein levels. Therefore, VRK2 down-regulation reduces cell invasion by tumor cells, such as MDA-MB-231 and MDA-MB-435, upon stimulation with phorbol 12-myristate 13-acetate plus ionomycin. These findings identify the first reported target and function of human VRK2 as an active kinase playing a role in regulation of cancer cell invasion through the NFAT pathway and COX-2 expression.

Obtusilactone B from Machilus Thunbergii Targets Barrier-to-Autointegration Factor to Treat Cancer

Targeting specific molecules is a promising cancer treatment because certain types of cancer cells are dependent on specific oncogenes. This strategy led to the development of therapeutics that use monoclonal antibodies or small-molecule inhibitors. However, the continued development of novel molecular targeting inhibitors is required to target the various oncogenes associated with the diverse types and stages of cancer. Obtusilactone B is a butanolide derivative purified from Machilus thunbergii. In this study, we show that obtusilactone B functions as a small-molecule inhibitor that causes abnormal nuclear envelope dynamics and inhibits growth by suppressing vaccinia-related kinase 1 (VRK1)-mediated phosphorylation of barrier-to-autointegration factor (BAF). BAF is important in maintaining lamin integrity, which is closely associated with diseases that include cancer. Specific binding of obtusilactone B to BAF suppressed VRK1-mediated BAF phosphorylation and the subsequent dissociation of the nuclear envelope from DNA that allows cells to progress through the cell cycle. Obtusilactone B potently induced tumor cell death in vitro, indicating that specific targeting of BAF to block cell cycle progression can be an effective anticancer strategy. Our results demonstrate that targeting a major constituent of the nuclear envelope may be a novel and promising alternative approach to cancer treatment.

Vaccinia-related Kinase 1 (VRK1) Is an Upstream Nucleosomal Kinase Required for the Assembly of 53BP1 Foci in Response to Ionizing Radiation-induced DNA Damage

Cellular responses to DNA damage require the formation of protein complexes in a highly organized fashion. The complete molecular components that participate in the sequential signaling response to DNA damage remain unknown. Here we demonstrate that vaccinia-related kinase 1 (VRK1) in resting cells plays an important role in the formation of ionizing radiation-induced foci that assemble on the 53BP1 scaffold protein during the DNA damage response. The kinase VRK1 is activated by DNA double strand breaks induced by ionizing radiation (IR) and specifically phosphorylates 53BP1 in serum-starved cells. VRK1 knockdown resulted in the defective formation of 53BP1 foci in response to IR both in number and size. This observed effect on 53BP1 foci is p53- and ataxia-telangiectasia mutated (ATM)-independent and can be rescued with VRK1 mutants resistant to siRNA. VRK1 knockdown also prevented the activating phosphorylation of ATM, CHK2, and DNA-dependent protein kinase in response to IR. VRK1 activation in response to DNA damage is a novel and early step in the signaling of mammalian DNA damage responses.